ironment upon animals. One of the fundamental characters of the
living protoplasm (using the word now in its widest sense) is its
extreme instability. So unstable is it that any disturbing influence
will affect it. If two similar unicellular organisms are placed under
different conditions they become unlike, since their unstable protoplasm
is directly affected by the surrounding conditions. With higher animals
the process is naturally a little more complicated; but here, too, they
are easily understood as part of the function of the machine. One of the
adjustments of the machine is such that when any organ is used more than
usual the whole machine reacts in such a way as to send more blood to
this special organ. The result is a change in the nutrition of the organ
and a corresponding variation in the individual. Thus acquired
variations are simply functions of the action of the machine.

Congenital variations, however, can not receive such an explanation.
Being born with the individual, they can not be produced by conditions
affecting him, but rather to something affecting the germ plasm from
which he sprung. The nature of the germ plasm controls the nature of the
individual, and congenital variations must consequently be due to its
variations. But it is not so easy to see how this germ plasm can
undergo variation. The conditions which surround the individual would
affect its body, but it is not easy to believe that they would affect
the germinal substance. Indeed, it is not easy to see how any external
conditions can have influence upon this germinal material if it is not
an active part of the body, but is simply stored within it for future
use in reproduction. How could any changes in the environment of the
individual have any effect upon this dormant material stored within it?
But if we are correct in regarding this germ material in the
reproductive bodies as the basis of heredity and the guiding force in
development, then it follows that the only way in which congenital
variations can occur is by some variations in the germ plasm. If a child
developed from germ plasm _identical_ with that from which its parents
developed, it would inherit identical characters; and if there are any
congenital variations from its parents, they must be due to some
variations in the germ plasm. In other words, in order to explain
congenital variations we must account for variations in the germ plasm.

Now, there are two methods by which